This blog provides a commentary on landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.The blog is written on a personal basis by Dave Petley, who is the Wilson Professor of Hazard and Risk in the Department of Geography at Durham University in the United Kingdom.

This blog is a personal project that does not seek to represent Durham University.

Wednesday, 30 July 2008

Well, I am finally back - apologies for the prolonged absence! It will take me a day or two to get back up to speed, but in the meantime I thought I'd post four landslide images from Switzerland and France that I took whilst on holiday (I have a very long suffering family...). In each case you can click on the image for a better view. Please feel free to use them in presentations, lectures etc but not commercially.

A large, apparently recent, complex rock fall near to Champery in Switzerland.

Wednesday, 9 July 2008

I am on my holiday from 9th to 28th July, so there will be few posts for the next couple of weeks. Of course, I have absolutely no intention of looking at any landslides while I am in Switzerland, honest guv...

Fifty years ago today, on the 9th July 1958, one of the most remarkable landslide events in recorded history occurred in Alaska. This was the Lituya Bay landslide, a large rockslide that collapsed catastrophically into a fjord in Alaska. Whilst the landslide itself was comparably unexceptional, though very large, the tsunami that it triggered most certainly was not.

Lituya Bay is located in the very southwest of Alaska (Figure 1). It consists of a narrow fjord some 12 km long and 2-3 km wide (Fig. 2). On the day in question at 10:16 pm, a magnitude 7.7 earthquake occurred on the Fairweather Fault about 21 km from Lituya Bay.

The earthquake triggered a collapse of a rockmass on a steep, recently debuttressed slope at the toe of the glacier at the top of the fjord (Fig. 3). The rockfall was large (probably about 40 million cubic metres) and catastrophic, falling as a single coherent mass into the fjord from a height of about 900 metres (fig. 4).

Fig. 3: Google Earth perspective view of Lituya Bay, highlighting the location of the rockfall. Click on the image for a better view (opens in a new window).

The fall triggered a wave in the fjord that raced from the landslide to the mouth of the fjord. The descriptions of this wave are remarkable, but are quite well shown in Fig. 5. The wave had a maximum run-up height (this is the vertical distance that it ran up the valley wall) of 530 metres. Whilst this sounds extreme, there is clear evidence that this was the case from sediments left by the wave and from the removal of trees by the water. This is the highest coastal wave ever recorded, although this very high run-up zone might be considered to me more of a splash than a coherent wave. The wave then travelled the length of the fjord, probably at a speed of about 150 km/hour, removing trees from above the waterline as it went. This removal of trees can be very clearly seen in Fig. 4 and is shown in more detail in Fig. 5. Whilst the height of the wave declined rapidly as it travelled down the fjord, it was still over 30 m high when it reached the mouth of the bay.

Fortunately of course Lituya Bay is located in a remote and essentially unsettled part of Alaska. There were however three small boats in the bay at the time of the landslide, the Edrie, the Badger and the Sunmore. The Edrie was at anchor when the wave, at that point about 30 metres high, struck her. Fortunately she rode over the wave and did not sink. The Badger was carried over the spit at the entrance to the fjord (see Fig. 4) and deposited in the open ocean, whereupon she sank. Fortunately the crew survived. The Sunmore tried to outrun the wave but was caught by it and engulfed. The boat and her crew were lost.

Tuesday, 8 July 2008

In 2006 Hans Rudolf Keusen of Geotest raised concerns that a large pillar on the Eiger mountain in Switzerland had become unstable. Over the next few weeks the pillar did indeed progressively fail, providing some amazing images of a large rockfall in action (Fig 1):

Figure 1: AP image entitled: On July 13, 2006, masses of rock fell down from Eiger, near Grindelwald. The event came after days of warnings from scientists regarding rock loosened by melting glacial ice.

Yesterday, Hans Rudolf Keusen again warned of the dangers of summer rockfalls on the Eiger. Once again, rockfalls have been occurring on the northwest side of the mountain, associated with the melting of the Lower Grindelwald glacier, which has left the flank of the mountain unsupported. The concern this time is that a rockfall could fall into a small lake at the toe of the slope, displacing "up to 900,000 cubic metres of water...in the space of several hours" (see here). Fortunately, monitoring systems are in place to detect any sudden rises in lake level, so a warning should be available.

Fig 2. Swiss Glacier Monitoring Network image of the Lower Grindelwald glacier, showing the debuttressed slopes above the moraines deposited by the glacier as it retreated.

The magnitude of the problem is well-illustrated by annual monitoring data that they have collected on the length of the glacier (Fig. 3). This shows the dramatic retreat of the glacier over a century, in which more than a kilometre has been lost. Unfortunately the data only extends to 1983, but the current average retreat rate is about 20 m per year.

First, the conference conveners should be congratulated for a well-organised meeting. The venue (the Shaanxi Guest House in Xian) was quite good, although I was quite glad that we chose to stay in town in the very welcoming Hyatt Hotel. It was great to meet up with old friends and colleagues, and to make some new friends too. Finally, the proceedings volumes are splendid - the editors have done an amazing job putting this together.

Turning now to the science. A real positive was the research presented by the delegates from China. In particular, the "China afternoon" was a great success, highlighting the range of projects being undertaken within the country, and the slope challenges associated with them. They are certainly not daunted by the complexity of the environment in which they work. I do feel that at times the research might benefit from input from experienced landslide scientists from elsewhere, and vice-versa. Hopefully this will come. In particular, at times the interpretation of the landforms might usefully benefit from some input, as might the understanding of the mechanisms of failure. The loess landslides are a case in point, in which both the interpretation of the morphology of the landslides themselves and the processes through which they have occurred would benefit from some attention.

I came away with a greater sense of unease regarding the research presented from outside China. Thinking back, I find it quite hard to think of many presentations that made me go "wow!". Often the research appears to be very focused on a single area or even a single landslide with little thought of the wider implications. Case studies are interesting and of value, but the real worth comes when the example is used to tell us something about landslides in general. A few people managed this, but in my view quite a few did not.

One thing that is clear is that a range of great data is now being collected through monitoring. The technologies for collecting such data, some of it in real time, have advanced incredibly over the last few years, providing insights into the details of landslide movement in time and space, and the controls upon it, in ways that have never been possible before. The availability of, for example, slope monitoring radar, terrestrial LIDAR, InSAR, etc is now really bearing fruit. I would like to see some more detailed analysis of the data (I think the technology is running ahead of the analysis at the moment), linked in to modelling and laboratory testing. Hopefully this is just a matter of time.

There was also a fair number of presentations on the application of models to replicate landslide processes, including both two and three dimensional simulations. There is some great work in this area as the codes and hardware become increasingly able to deal with the complexities of real systems. At times I felt a little frustrated that the outcome of the work seemed to be a demonstration that through careful tuning of the model parameters it was possible to replicate some aspect of the real system, such as the boundaries of the deposit or the estimated rate of movement of the landslide. In my view this is not enough - modelling is only useful if it helps us to understand the processes better and/or it allows us to forecast future behaviour. I am also struck by the lack of linkage between model outputs and field evidence - surely comparing the outputs of runout models with the the deposit thickness and where possible its structure is an sensible thing to do? These comparisons can be used to see if the model is really able to recreate what we see in the field, and then can be used to start to help us to understand landslide processes. Like many others, I worry that traditional field data collection is declining.

A further issue of concern is that lack of presentations on landslides in less developed countries, where many of the most serious impacts occur. There was almost nothing presented on landslides in India, Nepal, the Philippines, Indonesia, Colombia, Haiti, Pakistan, and so on. It appears that a huge range of landslide environments are not getting the research that they need, compounded of course by the costs of attendance for people from those countries. This is an issue that needs serious thought and attention.

Finally, I would like to point out a few presentations that I thought were genuinely superb. On the first day, Jordi Corominas from Barcelona gave a fantastic presentation on the linkage between pore pressures and movement rates. This showed that the relationship between the two is not simple, which provides a critical insight into the complexity of landslide systems. The most impressive keynote was that of Erik Eberhardt from UBC, who frankly never gives a bad paper. He integrated modelling and monitoring data to try to understand the movement of complex landslides. This work is right on the cutting edge and continues to impress. On the first day, Professor Zu-yu Chen presented a review of the landslide dam problems associated with the Wenchuan (Sichuan) earthquake, which was an incredibly understated description of an extraordinary achievement. Finally, Meei-Ling Lin from National Taiwan University reviewed the landslides triggered by the Chi-Chi earthquake nine years ago, highlighting the ways in which landslide problems associated with the 'quake were compounded by subsequent rainfall events. This is poorly understood, but vastly important.

So those are my impressions. I would be really interested to hear what other people who attended thought as well. Please do leave a comment!

Thursday, 3 July 2008

Tomorrow (Friday) I am due to present a paper at the 10th International Symposium on Landslides and Engineered Slopes. The paper title is: "Temporal prediction in landslides – understanding the Saito effect". The power point file is below (there appears to be a fault on slideshare that is preventing this from being visible on blogger. If it is not visible below, you can get it from here):

Wednesday, 2 July 2008

The map below shows the distribution of fatal landslides for June 2008. The statistics are:Number of fatal landslides: 43Number of fatalities: 215

As has been the pattern all year, June was substantially below the average number of fatalities for may in 2003-2007, which is 334 deaths. Interestingly, during June there have been extensive reports of intense and prolonged rainfall events, but this does not seem to have translated into fatal landslides. The reasons for this are unclear to me.

June 2008 fatal landslide locations (click for a larger version)

The patterns that were starting to emerge in the annual map over the last few months are now clear. In particular the clusters in the Himalayas, western Central America, the Philippines and Indonesia are now very obvious. Whereas last month the cluster in the Himalayas was focused on the western part of the mountain chain, it has now extended to the east as the monsoon rainfall has intensified. The pattern of more landslides in Central America than is normal is continuing. China is continuing to get a fair number of fatal landslides. It is also interesting to see the occurrence of several landslides in SE. Europe.

Recorded fatal landslides in 2008 up to the end of June. Each dot represents a single landslide. Click on the map for a larger version in a new window.

Tuesday, 1 July 2008

Today I participated in a round table discussion at the 10th International Symposium on Landslides and Engineered Slopes. The topic of the discussion was: "Strength criteria and design approaches for difficult soils and rocks".Each of the four panelists was asked to talk for 10-12 minutes before the panel discussion started. I was asked to talk about "Slow Moving Rockslides". So, below is my deliberately somewhat provocative presentation. (there appears to be a fault on slideshare that is preventing this from being visible on blogger. If it is not visible below, you can get it from here)

Hunza Landslide monitoring site

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